The invention relates to a current mirror in which an input current (Iin) is nonlinearly converted into a voltage in a current sink, the voltage being used for driving a current source (Iout) having substantially the same transfer characteristic.
The invention also relates to a method for operating a current mirror.
Current mirrors are circuits or circuit components that, as a response to a current, generate a current of the same intensity.
The prior art uses current mirrors, in particular, in integrated circuits in which data are processed based upon current signals.
A current mirror is, preferably, a circuit or a part of a circuit. It converts an input current into an output current. Preferably, there is a linear relationship between the input current and the output current.
Existing in the prior art is the use of suitably connected transistors as a current sink or as a current source.
Various circuit configurations for current mirrors are described in the book by C. Toumazou, F. J. Lidgey, and D. G. Haigh: xe2x80x9cAnalogue IC design: the current mode approachxe2x80x9d, London 1990.
It is particularly in highly accurate integrated analog circuits that differential signal processing is normally used for reducing the sensitivity of the circuits to interfering influences of the most varied type. While such is achieved by using fully differential operational amplifiers (high CMRR, PSRR, etc.) in circuits that transmit information in the voltage domain, current-mode circuits must use other solutions. In these circuits, quasi-differential current signal processing is frequently used in which two identical but electrically separate circuits are used in parallel. Due to the lack of direct electrical coupling and the possibly large spatial separation of the two signal paths on the chip, the advantages of differential signal processing become relative.
Other prior art embodiments are, in most cases, based upon using differential stages that result in disadvantages with respect to component mismatch and speed and output voltage range of the current mirror.
It is accordingly an object of the invention to provide a current mirror and method for operating a current mirror that overcome the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and that creates a current mirror that has the highest possible stability, particularly with respect to common-mode signals.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a method for operating a differential current mirror circuit, including the steps of generating, with devices, at least one copy of a respective one of a positive input current and a negative input current by respectively converting the respective one of the positive input current and the negative input current into a respective voltage, utilizing a respective voltage for respectively driving one current source to generate a copy of a respective one of a positive output current and a negative output current, generating, with a device, one auxiliary current that is one of a multiple and a fraction of each respective one of the positive input current and the negative input current, extracting, with a device, an interference signal by adding the two auxiliary currents, and providing one device each to generate the positive output current and the negative output current by subtracting the interference signal from a respective one of a copy of the positive input current and a copy of the negative input current.
With the objects of the invention in view, there is also provided a method for operating a differential current mirror circuit, including the steps of generating, with first devices, at least one copy of a respective one of a positive input current and a negative input current by respectively converting the respective one of the positive input current and the negative input current into a respective voltage, generating, with a second device, one auxiliary current that is one of a multiple and a fraction of each respective one of the positive input current and the negative input current, extracting, with a third device, an interference signal by adding the two auxiliary currents, and utilizing a respective voltage for respectively driving one current source to generate a copy of a respective one of a positive output current and a negative output current by providing one device each to generate the positive output current and the negative output current by subtracting the interference signal from a respective one of a copy of the positive input current and a copy of the negative input current.
In accordance with another mode of the invention, there is provided the step of generating the two auxiliary currents as one half of the respective one of the positive input current and the negative input current.
In accordance with a further mode of the invention, the device for generating the positive output current and the negative output current has at least one complementary current mirror.
In accordance with an added mode of the invention, there is provided the step of generating a multiple of the copy of the respective one of the positive input current and the negative input current respectively as the positive output current and as the negative output current.
In accordance with an additional mode of the invention, there is provided the step of generating a number of copies of the respective one of the positive input current and the negative input current.
With the objects of the invention in view, there is also provided a differential current mirror circuit, including devices generating at least one copy each of a respective one of a positive input current and a negative input current by respectively converting the respective one of the positive input current and the negative input current into a respective voltage, a respective current source connected to the devices, the respective voltage driving each current source to generate a copy of a respective one of a positive output current and a negative output current, a device connected to a respective one of the current sources and to the devices and respectively generating one auxiliary current that is one of a multiple and a fraction of the respective one of the positive input current and the negative input current, a device connected to a respective one of the current sources and to the devices and extracting an interference signal by adding the two auxiliary currents, and devices each generating the positive output current and the negative output current by subtracting the interference signal from a respective one of a copy of the positive input current and the negative input current.
With the objects of the invention in view, there is also provided a differential current mirror circuit, including first means for generating at least one copy each of a respective one of a positive input current and a negative input current by respectively converting the respective one of the positive input current and the negative input current into a respective voltage, a respective current source connected to the devices, the respective voltage driving each current source to generate a copy of a respective one of a positive output current and a negative output current, second means for respectively generating one auxiliary current that is one of a multiple and a fraction of the respective one of the positive input current and the negative input current, the second generating means connected to a respective one of the current sources and to the devices, means for extracting an interference signal by adding the two auxiliary currents, the extracting means connected to a respective one of the current sources and to the first means, and third means for respectively generating the positive output current and the negative output current by subtracting the interference signal from a respective one of a copy of the positive input current and the negative input current.
In accordance with yet another feature of the invention, the device or second means generates the two auxiliary currents as one half of the respective one of the positive input current and the negative input current.
In accordance with yet a further feature of the invention, the device for generating the positive output current and the negative output current or third means has at least one complementary current mirror.
In accordance with yet an added feature of the invention, the devices each generating the positive output current and the negative output current or third means generate a multiple of the copy of the respective one of the positive input current and the negative input current respectively as the positive output current and as the negative output current.
In accordance with yet an additional feature of the invention, the devices or first means generate a number of copies of the respective one of the positive input current and the negative input current.
The invention includes a fully differential symmetric current mirror unit, the two signal paths of which being direct-coupled. Such a configuration results in advantages in the rejection of interference signals.
According to the invention, a xe2x80x9cvoltage controlled current sourcexe2x80x9d is any component or any module that generates an output current when an input voltage is applied. In the simplest case, this can be an MOS transistor having a voltage applied to the gate thereof, a current being produced at the source-drain terminals. It is also conceivable to use a bipolar transistor. Modules of two transistors, three transistors, or even more complex voltage-controlled current sources are also conceivable.
According to the invention, the current mirror contains another voltage-controlled current source that supplies an auxiliary current (axc2x7Iout=axc2x7nxc2x7Iin).
The term xe2x80x9ccommon-mode rejectionxe2x80x9d is used in conjunction with differential amplifiers or operational amplifiers and relates to their characteristic of rejecting a common mode signal present at the input of the circuit. At the outputs of the circuit, only the differential signal appears that, e.g., is amplified.
The same is meant by common mode rejection in current mirrors. Independently of the common mode component at the inputs of the current mirror circuit, only the differential signal should appear at the output of the current mirror according to the invention.
The parameter a designates the fraction of the output current of the current mirror circuit that is provided as auxiliary current in order to correct for the interference signal in the differential current mirror circuit according to the invention.
In particular, the invention provides for constructing a fully differential current mirror without using differential stages that has advantages with regard to output voltage range, component mismatch and stability or bandwidth of the overall circuit.
The invention includes a symmetric current mirror unit in which an interference signal (common-mode component) of the differential signal is subtracted from the output current by a suitable supplementary circuit.
Current mirrors using MOS transistors are based on the fact that an input current (I+in=I0 or, respectively, Ixe2x88x92in=xe2x88x92I0) is nonlinearly converted in a current sink into a voltage that is used for driving a current source (I+out=xe2x88x92nxc2x7I0, or, respectively, Ixe2x88x92out=nxc2x7I0) having the same transfer characteristic. The current mirror thus formed is, then, extended by another voltage-controlled current source that supplies a current of axc2x7Iout+=xe2x88x92axc2x7nxc2x7I0. Such an auxiliary current is connected to the corresponding auxiliary current source of the identical second differential branch that delivers an output of axc2x7Ioutxe2x88x92=axc2x7nxc2x7I0. In such a case, the two currents cancel. If there is asymmetry in the input signal (common-mode component), the auxiliary currents will not cancel and the resultant residual current trigger xcex4I can be subtracted from the differential output signals with the aid of another current mirror having a transfer ratio of xc2xda. This results in mathematically complete common-mode rejection.
It is suitable to configure the current mirror such that an additional voltage-controlled current source in an additional branch generates an additional auxiliary current.
Advantageously, the current mirror is configured such that the auxiliary current and the additional auxiliary current are added such that the result is xcex4I.
In practice, it is advantageous to select:   a  =                    1        2            ⁢              xe2x80x83            ⁢      o      ⁢              xe2x80x83            ⁢      r      ⁢              xe2x80x83            ⁢      a        =    1  
because the entire current mirror can, then, be constructed by using identical n-MOS transistors or, respectively, p-MOS transistors.
Advantages of the circuit are the simple implementation as a supplementary module for existing current mirror units and the general independence of the architecture presented from the basic structure of the current mirror used. Furthermore, there are no stability problems due to the lack of feedback in the circuit so that the circuit is particularly suitable for applications with a wide signal bandwidth.
It is an important advantage of the invention to be able to implement fully differential current mirror structures without using a differential stage (see exemplary embodiment) that has various disadvantages with respect to bandwidth, output voltage range, and sensitivity to component mismatch.
It is appropriate that at least one of the current sources is formed by at least one transistor and/or that at least one of the current sources contains cascaded transistors.
According to the invention, the following distinguishing terms are used. In normal signal processing in circuits, the information to be processed is represented by an electrical signal (generally a voltage). Referred to a reference signal (e.g., ground), the magnitude of this signal is obtained from the difference between the (fixed) reference signal and the signal on the signal line. Such a type of signal processing based on absolute values is generally called xe2x80x9csingle ended.xe2x80x9d
In differential (=fully differential) signal processing, in contrast, the information is transmitted on two lines. Line 1 conducts the signal, line 2 conducts the inverted signal. The information represented by the signals is obtained by forming the difference between the two signal lines. The significant advantage of this differential signal processing is the clearly reduced sensitivity to induced interference from adjacent lines or from lines crossing the signal lines. In the xe2x80x9csingle-endedxe2x80x9d method, an interference signal induced in this manner is added to the useful signal and is processed further in the circuit. In differential signal processing, the interference is added on both signal-conducting lines. By forming the difference at the output of the circuit, the interference component of the signal is, thus, eliminated again. The differential architecture is particularly suitable for circuits in which very small signals must be processed or very accurate signal processing must be effected. Further information relating to this subject can be found in: xe2x80x9cDesign of analog integrated circuits and systemsxe2x80x9d by Kenneth R. Laker and Willy M. C. Sansen, McGraw-Hill publishers, ISBN 0-07-036060-X, page 456ff.).
In this context, the term xe2x80x9cquasi-differential current mirrorxe2x80x9d designates a current mirror circuit in which two signal paths are provided for the signal and the inverted signal but interference is not eliminated within the circuit. Thus, the interference is forwarded to the stage following the current mirror and can lead to an impairment of the signal (e.g., due to saturation effects) there, or in a later stage in the signal processing chain.
The terms xe2x80x9cdifferential,xe2x80x9d xe2x80x9cfully differential,xe2x80x9d and xe2x80x9cfully differential symmetric current mirrorxe2x80x9d designate the same circuit architecture. A current mirror circuit is constructed here that has two signal pathsxe2x80x94for the noninverted signal stream and the inverted signal stream. Furthermore, the common-mode component (=interference signal) is cancelled within the circuit so that only the useful signal is present at the output of the current mirror.
The term xe2x80x9cdifferential signal processingxe2x80x9d can be found in xe2x80x9cDesign of analog integrated circuits and systemsxe2x80x9d by Kenneth R. Laker and Willy M. C. Sansen, McGraw-Hill publishers, ISBN 0-07-036060-X, page 456ff.). The term xe2x80x9cquasi-differentialxe2x80x9d is frequently used by the applicant in connection with this subject.
If the device for generating the positive output current (Iout+) and the negative output current (Ioutxe2x88x92) by subtracting the interference signal (Icor) from a copy of the positive input current (Iin+) or, respectively, from a copy of the negative input current (Iinxe2x88x92) has at least one complementary current mirror (M11, M14; M12, M15; M13, M16), then both positive and negative interference signals can be subtracted.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a current mirror and method for operating a current mirror, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.